Photolithography is an essential process in semiconductor device and integrated circuit fabrications. Because of their high resolution, phase shift photo masks are often used in fabricating semiconductor devices and integrated circuits of fine size. FIG. 1 shows a schematic cross sectional view of a phase shift photo mask 10 suitable for a photolithography process in the fabrication of fine size semiconductor devices and integrated circuits. Phase shift photo mask 10 is formed from an optically transparent substrate 11. Opaque light blocking structures 14, 16, 17, and 19 are formed on a major surface 12 of substrate 11. Typically, substrate 11 is made of quartz glass and light blocking structures are chromium structures. Structures 14, 16, 17, and 19 define a pattern to be transferred to a semiconductor wafer in the photolithography process. Substrate 11 is selectively etched to form depressions or cavities 15 and 18. Cavity 15 changes the optical thickness of substrate 11 there under. Consequently, the lights transmitted through substrate 11 near two opposite sides of light blocking structure 14 have a phase shift of 180 degrees relative to each other. This 180.degree. phase shift creates an interference pattern on the semiconductor wafer and significantly increases the image resolution of light blocking structure 14 on the semiconductor wafer. Cavity 15 also increases the image resolution of light blocking structure 16. Likewise, cavity 18 increases the image resolutions of light blocking structures 17 and 19. Cavities 15 and 18 are also referred to as phase shifters.
A way of achieving high quality, reliability, and yield of the photolithography processes is inspecting photo mask 10, and repairing or discarding photo mask 10 if it has defects. Photo mask 10 is usually inspected using an optical microscope. Because quartz glass is colorless and transparent to light over a wide spectrum, the light transmittance of substrate 11 is nearly 100 percent and substantially independent of its thickness. Therefore, determining the existence, location, and dimension of cavities 15 and 18 on substrate 11 is difficult and time consuming. The inspection of photo mask 10 is complicated and often fails to accurately reveal the status and condition of phase shifters 15 and 18 on photo mask 10.
Accordingly, there exists a need for a phase shift photo mask that can be easily and accurately inspected for possible defects. There is also a need for methods of making and inspecting such a phase shift photo mask. It is desirable for the methods to be simple and cost efficient. It is also desirable for the phase shift photo mask to be compatible with existing photolithography processes.